Enhanced signal detection in a time-dispersive medium generally requires a receiver to perform some type of echo signal equalization on the received time-dispersed signals to produce an output which has a better output than would result from allowing the echoes to interfere with one another. One such equalization technique used in a digital radio Time Division Multiple Access (TDMA) system is described in instant assignee's U.S. Pat. No. 4,829,543 entitled "Phase-Coherent TDMA Quadrature Receiver for Multipath Fading Channels" filed on behalf of Borth et al.
The Borth et al. invention describes a phase coherent method for demodulating a Quadrature Phase Shift Keyed (QPSK) radio signal that is subjected to multipath fading. Equalization is facilitated by correlating a stored training sequence, known to the receiver, against the incoming signal, and using the resulting correlation to remove the phase difference between the incoming signal and the receiver's local oscillator, effecting coherent detection. Equalization can then proceed.
Other techniques have been proposed for dealing with the intersymbol interference which can be generated in a transmitted signal by a time-dispersive transmission channel. Such receivers are described in "Adaptive Maximum Likelihood Receiver for Carrier-Modulated Data-Transmission Systems", authored by G. Ungerboeck, IEEE Transactions on Communications, Vol. COM-22, No. 5, May 1974, pp. 624-636, and "Maximum Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", authored by G. D. Forney, IEEE Transactions on Information Theory, Vol IT-18, No. 3, May, 1972, pp. 363-377.
However, in high data rate systems where transmission is through a severely delay-spread radio channel, single branch-single receiver equalization may fail to provide adequate time-dispersed distortion (multi-ray fading) correction. For example, practical implementations of equalizing receivers may have imperfect estimates of the critical error signal in the case of decision feedback equalization, or imperfect estimates of the transmission channel's impulse response in some other equalization schemes.
Therefore, diversity reception (the same signal received on multiple branches--which may be on different antennas, or on a single antenna at different times, or made in other ways, as is well known in the art) is typically necessary to sufficiently reduce the effect of multi-ray fading. One such receiver is described in U.S. Pat. No. 4,271,525 entitled, "Adaptive Diversity Receiver For Digital Communications". This patent describes an adaptive diversity receiver using an adaptive transversal filter for each receiver branch, followed by a decision feedback equalizer. The tap gains of the transversal filters are updated via feedback from the output of the equalizer, and other points in the receiver.
U.S. Pat. No. 4,731,801 entitled "Method For The Reception And Detection Of Digital Signals" discloses an improvement over U.S. Pat. No. 4,271,525 and other prior art by improving reception in highly dispersive transmission paths using coherent demodulation. This invention uses a technique wherein the output of the bit decision circuitry becomes a basis for calculating a correction signal. A reference carrier, resulting from summing the quadrature baseband signals and the in-phase baseband signals, is fed back to the local oscillator of quadrature demodulators which in turn compensates the phase difference between the received signals and the receiver's local oscillator to facilitate coherent demodulation.
However, inventions such as described in U.S. Pat. No. 4,271,525 require a set of adaptive transversal filters, one for each receiver branch, in addition to the equalization circuitry. Inventions such as U.S. Pat. No. 4,731,801 require complex circuitry to phase shift the signal in each diversity branch, and, more importantly, cannot arrive at the correct phase adjustments quickly enough to be useful in, for example, TDMA systems characterized by information which is received, and must be corrected, in short bursts separated by relatively long periods of time. During these long periods, signal phases in multi-ray fading channels can change radically relative to the receiver's local oscillator.
Accordingly, there exists a need for a reduced complexity receiver that performs diversity reception on continuous, or non-continuous, high speed digital signals and is capable of substantially reducing effects of both flat fading and multi-ray, dispersive fading due to time-dispersive transmission mediums.